Traditional water level gauging instruments are generally in contact with the water itself (susceptible to biofouling) or include relatively expensive non-contact equipment that is relatively inaccurate when used.
Some traditional water level gauging method consists of a massive station house, a gauge shaft and a measuring unit. It is costly to build up the station and requires a lot of permanent maintenance.
New technologies have been introduced for water level gauging, such as Radar, ultrasonic, and laser. Ultrasonic has low accuracy due to its long wavelength resolution. Laser sensors are costly and not easy to be installed for long-term gauging. Radar sensor is a more attractive approach.
Existing radar level gauging techniques mainly include pulse radar and frequency modulated continuous wave (FMCW) radar. However, pulse radar is usually not desirable due to the low measurement accuracy unless an expensive data converter is used. FMCW radar, on the other hand, suffers from multipath echoes reflected from various objects around the water body, which limits the radar resolution and is also inaccurate.
Microwave radar has become an attractive approach for noncontact displacement and distance measurement. The conventional Doppler radar technique has been used for small-scale displacement detection, e.g., vital signs of respiration and heartbeat (amplitude is less than a few cm). The small displacement is negligible compared to the distance between the subject and the radar. Therefore, the amplitudes of the measured signals and the DC offset at RF output are almost constant, and typically the phase modulation does not exceed 180 degrees (half carrier wavelength). Arctangent demodulation with prefixed DC calibration is sufficient to recover the phase information of the small-scale displacement. However, if the displacement is so large that it is comparable to the distance between the subject and the radar, e.g. in the case of radar water level gauging, it would significantly affect the power received at the radar input and the baseband signals would be subject to inconstant amplitude and dynamically varying DC offset. Moreover, the large displacement inevitably leads to phase ambiguity in the conventional arctangent demodulation.
While many radar gauging devices are research projects in academic institutions, commercial businesses can include Campbell Scientific, which provides a pulse radar sensor that is subject to lower accuracy in water level displacement measurement compared to CW radar; OTT RLS Radar Level Sensor product that has low accuracy of 0.01 ft and measures water level every 20 seconds—not continuously; VAISALA is another example of a pulse radar sensor, subject to lower accuracy and higher hardware complexity. in water level displacement measurement in comparison to CW radar; and VEGAPULS which is based on ultrasonic signals.
To solve the problems with existing radar level gauging techniques, a water level gauging technique is needed that is based on a DC (direct current)-coupled CW Doppler radar sensor. Unlike the conventional AC-(alternating current) coupled Doppler radar sensor, which suffers from signal distortion when measuring the slow movement of water level motion, this radar gauging technique employs a DC-coupled architecture that allows accurate measurement of the slow-varying water level. It is also immune from the interference of multipath echoes, which exist in FMCW radar, because the clutter reflections from surrounding stationary objects only produce a DC offset at the RF (radio frequency) output. The DC offset can be easily compensated by the baseband adaptive-tuning architecture, and the technique has sub-millimeter accuracy.